Conductive adhesive and biomedical electrode

ABSTRACT

The conductive adhesive of the present invention has a multi-layer structure comprising a hydrophobic phase containing an adhesive polymer, and a hydrophilic phase containing an electrolyte. This conductive adhesive is superior in anti-drying effect (moisturizing effect) because a humectant is effectively contained in the hydrophilic phase. The present invention also relates to a biomedical electrode using such a conductive adhesive. In the biomedical electrode according to the present invention, an adhesive layer to be applied to the skin surface of the human body contains the conductive adhesive described above and an electrode terminal connected with the adhesive layer is provided.

FIELD OF THE INVENTION

The present invention relates a conductive adhesive which developsconductivity through the medium of an electrolyte contained therein and,more particularly, to a conductive adhesive suited for use as abiomedical electrode.

BACKGROUND OF THE INVENTION

A biomedical electrode is usually composed of a conductive layer to bemade in contact with the skin of mammals (including human) and anelectrode connected electrically with the conductive layer, and is usedin various applications depending on its construction. For example, abiomedical electrode can be used for electrical diagnosis, treatment orsurgery of mammals. As a device for measuring a cardiac actionpotential, for example, the biomedical electrode is applied to thesurface of the living body. In such a case, the biomedical electrodedetects a change in potential on the surface of the living body in anelectrocardiograph for measuring and recording a cardiac actionpotential. A change in potential on the surface of the living body,detected by such a biomedical electrode, is inputted into anelectrocardiograph equipped with the biomedical electrode, recorded asan information of an electrocardiogram, and then displayed or printed. Aterminal of the biomedical electrode is connected to theelectrocardiograph through a connection cable or lead wire.

The above-described biomedical electrodes can comprise, for example, anAg/AgCl conductor eyelet, a stud of carbon (or metal) as an electrodeterminal, and a conductive non-adhesive gel layer as a conductive layerto be placed in contact with the skin surface. Such a conductivenon-adhesive gel is disclosed, for example, in Japanese NationalPublication (Kohyo) No. 56501108; or corresponding U.S. Pat. No.4,406,827 and Japanese Utility Model Registration No. 2570183. It isnecessary that such a biomedical electrode further comprises an adhesivetape (backing tape) for fixing it to the surface of the living body.Biomedical electrodes that include a conductive adhesive layer as aconductive layer, however, do not require the adhesive tape.

A conductive adhesive, which can be used as the above-describedconductive adhesive layer, is disclosed in Japanese Examined PatentPublication (Kokoku) No. 8-19394 and U.S. Pat. No. 4,524,087. Abiomedical electrode utilizing a conductive adhesive is disclosed inU.S. Pat. No. 5,078,139. Such biomedical electrodes can be fixed to thesurface of the living body without containing a backing adhesive tapefor fixing, and can comprise:

(a) a conductive adhesive layer containing an aqueous electrolytesolution and an adhesive polymer,

(b) a liner for coating one surface (adhesive surface) of the conductiveadhesive layer,

(c) a backing for coating the other surface of the conductive adhesivelayer, and

(d) an electrode terminal connected with the conductive adhesive layer,which has an exposed portion coated neither with the liner nor backing.

When this biomedical electrode is applied to the skin, the electrode canbe readily fixed only by peeling off the liner to expose one surface ofthe conductive adhesive layer, and bringing the adhesive surface intocontact with the surface of the living body thereby to slightlycontact-bond them. A conventional conductive adhesive layer hassufficient initial adhesive strength, but as a result of absorption ofsweat from the skin into the adhesive layer the adhesive strength islikely lowered over time.

U.S. Pat. Nos. 5,779,632 and 5,670,557 disclose a so-called“bicontinuous conductive adhesive” having a continuous structurecomprising a hydrophilic conductive phase containing an aqueouselectrolyte solution and a hydrophobic adhesive phase. In such abicontinuous conductive adhesive, the hydrophilic phase containing anelectrolyte is a continuous layer and ionic conductivity can beexhibited. Since the hydrophobic adhesive layer has a bonding functionand sweat from the skin is absorbed by the hydrophilic phase, loweringof the adhesion strength can be improved. In the bicontinuous conductiveadhesive (or biomedical electrode using the same), however, it wasparticularly difficult to retain water content during storage under lowhumidity conditions. Accordingly, the biomedical electrode must bestored in a sealed pouch to prevent drying (vaporization) of the phaseincluding the aqueous electrolyte. It is also normally necessary to usethe biomedical electrode within 10 to 30 days after opening the sealedpouch since, it becomes difficult to obtain sufficient electricalcharacteristics (i.e., sufficiently low impedance) when the conductiveadhesive dries.

U.S. Pat. No. 5,338,490 discloses an adhesive comprising (A) a firstphase containing a hydrophilic polymer and an aqueous solution ofelectrolyte, and (B) a second phase containing a hydrophobic adhesivepolymer, wherein the first phase is a continuous phase and the secondphase is a domain phase contained in the state of being dispersed in thefirst phase.

U.S. Pat. No. 5,270,358 discloses a so-called “bidispersed” adhesivecomposition comprising a continuous phase of a hydrophobicpressure-sensitive adhesive and a hydrophilic dispersed phase of ahydrogel having pressure-sensitive adhesion characteristics. However,this adhesive composition has no ionic conductivity because thehydrophilic dispersed phase (each particle of hydrogen) is not acontinuous phase and, therefore, the adhesive composition can not beused as the conductive adhesive.

It is desirable to include a humectant in the conductive adhesive of abiomedical electode to impede drying of the conductive adhesive.However, when high-performance humectants such as amino acids are addedto the bicontinuous conductive adhesive to obtain a high moisturizingeffect, an expected two-phase structure can not be obtained. That is,when a comparatively large amount of the humectant exists, a continuousstructure of a hydrophobic adhesive phase is likely broken into anemulsion as a raw material of the conductive adhesive, thereby making itdifficult to maintain the structure in an effective state (e.g. a statewhere sufficient adhesion property can be exhibited).

SUMMARY OF THE INVENTION

In one aspect the present invention provides a conductive adhesivecomprising:

(A) a first phase containing a hydrophilic polymer, an aqueouselectrolyte solution and a humectant, and

(B) a second phase containing a hydrophobic adhesive polymer,characterized in that:

the first phase is a continuous phase and the second phase is a domainphase that is dispersed in the first phase, and the domain phase has anaverage diameter within a range from 0.02 μm to 1 mm.

In another aspect, the present invention provides a biomedical electrodecomprising an adhesive layer containing the conductive adhesive of thepresent invention, and an electrode terminal connected with the adhesivelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a planar view showing one preferred embodiment of a biomedicalelectrode according to the present invention.

FIG. 2 is a cross-sectional view showing a configuration of thebiomedical electrode shown in FIG. 1.

FIG. 3 is a planar view showing another preferred embodiment of abiomedical electrode according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The conductive adhesive and biomedical electrode according to thepresent invention can be advantageously carried out in various aspectsas described in detail below.

The conductive adhesives of the present invention is characterized, intheir essential aspects, by a conductive adhesive that comprises: (A) afirst phase containing a hydrophilic polymer, an aqueous electrolytesolution and a humectant (collectively referred to as the “hydrophilicphase” unless otherwise stated because it usually is hydrophilic) and(B) a second phase containing a hydrophobic adhesive polymer (referredto as a “hydrophobic phase” unless otherwise stated because it usuallyis hydrophobic), wherein the hydrophilic phase is a continuous phase andthe hydrophobic phase is a domain phase dispersed in the first phase. Insuch a dispersed adhesive having a two-phase structure, since thehydrophobic phase is originally a domain phase, it is easy to maintain astable phase structure in a comparatively wide composition range andprevent breakage of the phase structure. Accordingly, a humectant can beeffectively added to the continuous phase (i.e. the hydrophilic phase)to enhance the moisturizing effect and effectively prevent drying of theconductive adhesive (or conductive adhesive layer of the biomedicalelectrode). That is, according to the present invention, a humectanthaving a high moisturizing effect can be used.

In this dispersed adhesive having a two-phase structure, since thehydrophilic phase containing an electrolyte is a continuous phase, theionic conductivity is high and the adhesive is suitably used as theconductive adhesive.

An average diameter of the domain phase constituting the adhesive can beappropriately decided depending on desired performance characteristicssuch as humidity retention effect and the like. Average diameters withina range from about 0.02 μm to about 1 mm can be used. When the averagediameter of the domain phase is smaller than about 0.02 μm, thecomposition range capable of maintaining the phase structure may becomenarrow. Narrow composition ranges make it difficult to mix a requisiteamount of high-performance humectant (e.g. amino acids, etc.), therebylimiting the moisturizing effect. In other words, to incorporate arequisite amount of high-performance humectants (e.g. amino acids, etc.)to enable easy and stable retention of the phase structure, it issuitable to increase the average diameter of the domain phase to thepredetermined lower limit or more. On the other hand, when the averagediameter exceeds 1 mm, it becomes difficult to form and maintain thedomain phase and it is likely to become difficult to produce an adhesivehaving a uniform structure. When the structure of the adhesive becomesuniform, it becomes difficult to control the adhesion property andconductivity within a desired range. Accordingly, the average diameterof the domain phase is preferably within a range from about 0.03 toabout 100 μm, and more preferably from about 0.04 to about 10 μm.

Any known humectant composition may be used in the present invention.Generally, preferred humectants will be compatible with and soluble inthe hydrophilic phase of the conductive adhesive. Amino acids are apreferred class of humectant compounds.

The adhesive is preferably prepared in the following manner to enhance amoisturizing effect.

First step:

A stock solution is prepared that comprises:

(a) an aqueous medium containing a first monomer capable of forming ahydrophilic polymer after polymerization and a humectant composition,and

(b) a second monomer capable of forming a hydrophobic adhesive polymerafter polymerization, which is dispersed in the aqueous medium.

Second step:

The stock solution prepared in the first step is subjected to apolymerization treatment, to obtain a precursor syrup solution (as anadhesive precursor) that comprises:

(1) a continuous phase formed from the aqueous medium, and

(2) a domain phase, which is dispersed in the continuous phase of theaqueous medium and contains the hydrophobic adhesive polymer.

Third step:

The precursor syrup solution obtained in the second step can be furthersubjected to a polymerization treatment to form a conductive adhesive ofthe present invention.

Using the above preparation method, since a continuous and hydrophilicphase and a hydrophobic adhesive domain phase are formed uponpolymerization of each starting monomers, it becomes easy to chemicallybond the phases and effectively increase the stability of the resultingphase structure. Accordingly, it becomes possible to effectively utilizea humectant, (particularly an amino acid humectant) and thus effectivelyprevent undesired drying.

In the first step of the above-described preparation method, a dispersedphase containing a second monomer is usually dispersed in a continuousphase containing an aqueous medium by a mixing/dispersion operation suchas stirring. In the second step, all or portion of the second monomer ispolymerized to form a domain phase containing a hydrophobic adhesivepolymer. The dispersion stability of the domain phase containing thehydrophobic adhesive polymer thus prepared is very high, thereby makingit possible to effectively prevent the dispersed structure from breakingby the presence of the humectant. When the dispersed structure of thedomain phase is broken the adhesion property and conductivity arelowered.

It is easy to impart a comparatively low viscosity, which isadvantageous to conduct a coating or immersing operation, to theadhesive precursor syrup prepared in the second step. Accordingly, it isvery easy to further subject the precursor syrup solution formed as alayer by coating to a polymerization treatment in the third step,thereby forming a conductive adhesive layer. Accordingly, the adhesiveis particularly suited to form a conductive adhesive layer used as abiomedical electrode.

Another aspect of the present invention provides a biomedical electrodecomprising an adhesive layer containing a conductive adhesive, and anelectrode terminal connected with the adhesive layer. According to thisbiomedical electrode, the anti-drying effect (moisturizing effect) ofthe adhesive layer can be effectively enhanced without lowering theadhesion property and conductivity. Such a biomedical electrode can bemaintained for a long period of time without being dried even whenallowed to stand under a low-humidity condition outside a sealed pouch.The conductive adhesive of the present invention can also be used as acomposition for uses other than as an adhesive layer.

The conductive adhesive of the present invention, componentsconstituting the conductive adhesive, methods of preparing the adhesiveusing those components, biomedical electrodes and methods of producingthe same are now described below. It will be appreciated that theconductive adhesive and biomedical electrode of the present inventionare not limited by the following descriptions.

Conductive Adhesive

As described above, in the conductive adhesive of the present invention,a domain phase dispersed in a hydrophilic continuous phase (hydrophilicphase) comprises a hydrophobic adhesive polymer. Such a hydrophobicphase exists as a domain phase dispersed stably in a continuous phase ofa hydrophilic phase.

The above stable domain phase can be formed in the following procedure.

First, a stock solution is prepared that comprises:

(a) an aqueous medium containing a first monomer capable of forming ahydrophilic polymer after polymerization and a humectant,

(b) a second monomer capable of forming a hydrophobic adhesive polymerafter polymerization, which is dispersed in the aqueous medium, and

(c) an oil-soluble polymerization initiator.

The solution is then subjected to a polymerization treatment to form aprecursor syrup solution comprising a continuous phase formed from theaqueous medium and a domain phase, which is dispersed in the continuousphase and contains the hydrophobic adhesive polymer.

The above stock solution used for formation of the precursor syrupsolution as the adhesive precursor is usually prepared by dispersing adispersed phase containing the second monomer and oil-solublepolymerization initiator in the continuous phase containing the aqueousmedium by a mixing/dispersion operation such as stirring. In this way, acomparatively stable dispersed phase can be formed with an appropriatesurfactant contained in the stock solution or by utilizing appropriatestirring conditions.

In the case where the precursor syrup solution is subjected to apolymerization operation to obtain a conductive adhesive as finalproduct, it is comparatively difficult to form a domain phase containinga hydrophobic adhesive polymer having a predetermined size. Therefore,the stock solution subjected to this dispersion operation is preferablysubjected to a polymerization treatment (first polymerization treatment)to polymerize all or portion of the second monomer, thereby stabilizingthe domain phase containing the hydrophobic adhesive polymer. Thedispersion stability of the domain phase thus formed is very high,thereby making it possible to effectively prevent the dispersedstructure from breaking by the presence of the humectant. Then, awater-soluble polymerization initiator is added to the precursor syrupsolution formed as described above and is further subjected to apolymerization treatment (second polymerization treatment) to form aconductive adhesive of the present invention. In this stage,substantially all of the above first and second monomers arepolymerized. The hydrophilic polymer formed from the first monomerconstitutes a hydrophilic phase as a continuous phase in thefinally-obtained conductive adhesive. According to such a method, theconductive adhesive as the final product can be obtained while thedomain phase containing the hydrophobic adhesive polymer formed in theprecursor syrup solution is stably maintained.

Since the domain phase contained in the precursor syrup solution is verystable, a coating solution having the viscosity suited for coating canalso be formed by adding a diluting solvent such as water to theprecursor syrup solution. To crosslink the hydrophilic polymer and/orhydrophobic polymer, a crosslinking agent can also be added to theprecursor syrup solution before the final polymerization operation.

As described above, one embodiment of the present invention alsoprovides a preferred precursor syrup solution so as to form a conductiveadhesive. That is, there is provided a precursor syrup solution formedby subjecting a stock solution comprising:

(a) an aqueous medium containing a first monomer capable of forming ahydrophilic polymer after polymerization, and a humectant,

(b) a second monomer capable of forming a hydrophobic adhesive polymerafter polymerization, which is dispersed in the aqueous medium, and

(c) an oil-soluble polymerization initiator.

Non-indented to a polymerization treatment, said precursor syrupsolution comprising:

a continuous phase formed from the aqueous medium, and

a domain phase which is dispersed in the continuous phase and containsthe hydrophobic adhesive polymer. The precursor syrup solution is usedfor forming a conductive adhesive by further subjecting to apolymerization treatment.

Method of Producing Conductive Adhesive

In accordance with its preferred aspect, the adhesive of the presentinvention is composed of a domain phase of a hydrophobic adhesivepolymer, and a hydrophilic and ionic conductive continuous phase, andhas a white or opaque emulsion structure. The average diameter of thedomain phase (hydrophobic phase) is within a range from about 0.02 μm toabout 1 mm. Such an emulsion is usually referred to as a macro- ormicro-emulsion.

The structure of the macro- or micro-emulsion can be preferably formedin the following procedure. First, a precursor syrup solution (as anadhesive precursor) of an emulsion stabilized by UV irradiation orheating is formed, using a predetermined polymerization initiator andseparating by stirring the unstable raw material mixed solution (stocksolution). This stabilized emulsion syrup solution is subjected to UVpolymerization, thereby making it possible to obtain an adhesive as afinal product.

The size of the hydrophobic domain phase in the adhesive of the presentinvention can be controlled, for example, by adding a surfactant to thestock solution. Usually, the size of the domain phase depends on thekind of the surfactant, composition (formulation) of the adhesive andtechnique for conversion into a syrup (e.g. composition of syrup,stirring conditions, polymerization condition, etc.) and is controlledby appropriately selecting these factors.

To easily obtain a stable emulsion, the average diameter of thehydrophobic domain phase is preferably within a range from about 0.04 toabout 10 μm.

Preferably, the starting material for synthesizing the adhesive of thepresent invention includes:

(1) 2 to 50% by weight of water for forming a hydrophilic phase (thiswater also functions as a solvent which dissolves a salt for electrolytesolution described hereinafter to form an aqueous electrolyte solution),

(2) 5 to 40% by weight of a hydrophobic monomer (may also be a monomermixture) capable of forming an adhesive hydrophobic polymer in ahydrophobic phase (also referred to as hydrophobic phase),

(3) 1 to 40% by weight of a hydrophilic or amphiphatic monomer oroligomer which partially exists in a hydrophilic phase and is capable offorming a hydrophilic polymer,

(4) 0.01 to 30% by weight of a surfactant for easily forming a macro- ormini-emulsion,

(5) 5 to 50% by weight of a humectant of amino acids, which has a highmoisturizing effect,

(6) 0.5 to 5% by weight of salts for electrolyte solution,

(7) 0.01 to 10% by weight of a plasticizer,

(8) 0.01 to 1% by weight of a crosslinking agent,

(9) 0.05 to 2% by weight of a water-soluble free-radicalphotopolymerization initiator, and

(10) 0.005 to 1% by weight of a water-soluble free-radicalphotopolymerization initiator or an oil-soluble free-radicalphotopolymerization initiator. The proportion of the above raw materialcomponents is selected so that the total amount becomes 100% by weightas far as percentages are by weight.

The hydrophilic polymer is preferably an adhesive polymer. A macro- ormicro-emulsion conductive adhesive comprising an adhesive domain phaseand a continuous hydrophilic phase can be obtained by selecting the kindand proportion of the monomer or oligomer.

Usually, the hydrophilic phase (also referred to as an aqueous medium)contained in the stock solution contains:

water,

at least one free-radically copolymerizable ethylenically-unsaturatedamphiphatic or hydrophilic monomer or oligomer,

salty or salts as an electrolyte, and

a humectant.

The hydrophilic phase can contain other additives, for example,non-reactive polar oligomer additives, free-radically polymerizablephotoactivated crosslinking agents, auxiliary solvents, water-solublefree-radical photopolymerization initiators, water-soluble free-radicalthermopolymerization initiators, plasticizers and the like.

The hydrophobic phase of the stock solution contains, as a secondmonomer, a free-radically polymerizable hydrophobic monomer capable offorming a hydrophobic pressure-sensitive adhesive. The hydrophobic phasecan contain other arbitrary additives, for example, oil-solublefree-radically photopolymerization initiators, non-reactive polaroligomer additives, oil-soluble crosslinking agents, oil-solublefree-radical thermopolymerization initiators, plasticizers, tackifiersand the like.

The above-described macro- or micro-emulsion can be formed by utilizinga high-speed stirrer, a homogenizer, etc. without using a surfactant.Preferably, a nonionic, anionic, cationic or amphoteric surfactant isused. A reactive surfactant having an ethylenically-unsaturated bond canalso be used.

To form a stable macro- or micro-emulsion structure, a stock solutioncomprising the components other than the crosslinking agent andwater-soluble polymerization initiator among the above-described rawmaterial components is prepared first, and the stock solution issubjected to a polymerization treatment to polymerize all or portion ofthe hydrophobic monomer to form a precursor syrup solution. Othercomponents are usually mixed in a reactor and subjected to apolymerization treatment in the same reactor with stirring.

A stable domain phase having a diameter within a predetermined range canbe maintained for a long period of time (usually several hours or more)by such an operation for forming a syrup. The average diameter of thedomain in the opaque syrup solution thus obtained is usually measured bya laser scattered particle analyzer. Specific examples of the measuringdevice include a laser scattered particles analyzer LS230 (model No.)manufactured by Coulter Co.

The above-described crosslinking agent and water-soluble polymerizationinitiator are added to the syrup solution as a post additive. The syrupsolution obtained by adding the post additive is poured into a mold andthen subjected to a polymerization treatment. In such a manner, anadhesive as a fmal product can be obtained. Alternatively, a layer of aconductive adhesive can also be formed by coating the syrup solutioncontaining the post additive with a substrate to form a film, followedby a polymerization treatment. A porous substrate such as sheet or meshsheet containing scrim is impregnated with the syrup solution containingthe post additive and then subjected to a polymerization treatment,thereby making it possible to obtain an adhesive sheet, into which thesubstrate is incorporated as a reinforcing material.

One advantageous effect of the present invention is that an ionconductive adhesive comprising a domain phase containing stably ahydrophobic adhesive polymer in a continuous hydrophilic conductivephase containing a comparatively large amount of a humectant having ahigh moisturizing effect can be easily produced. According to thepresent invention, the limitation about the formulation of therespective components is substantially removed, thereby making itpossible to form and to maintain a stable macro- or micro-emulsionstructure even when a comparatively large amount of a humectant selectedfrom amino acids having a high moisturizing effect.

Hydrophilic Phase

The hydrophilic phase usually contains water in the amount within arange of about 2 to about 50% by weight, and preferably from about 25 toabout 40% by weight, based on the total weight of the conductiveadhesive. When the amount of water contained in the hydrophilic phase istoo small, the moisturizing performance and conductivity are likely tobe lowered. On the other hand, when the amount is too large, thecohesive force is lowered and the adhesion property is likely to belowered. Water is preferably deionized water.

Hydrophilic Polymer

The hydrophilic polymer may be those having no adhesion property.However, those having adhesion properties are preferred for thefollowing reason. In case where the conductive adhesive has a macro- ormicro-emulsion structure comprising a domain phase containing ahydrophobic adhesive polymer, and a continuous phase containing ahydrophilic adhesive polymer, both the adhesion strength on initialbonding can be enhanced. Since both of the hydrophobic and hydrophilicadhesive polymers are contained in different phases, the adhesionproperty is not lowered when the adhesion condition varies, for example,in case where the skin in contact with the adhesive causes sweating orwhere body exudate such as body fluid effuses from the skin.

The hydrophilic polymer is a polymer of a hydrophilic monomer oroligomer. For example, the first monomer as a raw material of anadhesive, which is contained in the aqueous medium of the stocksolution, is substantially composed of a hydrophilic monomer.

The hydrophilic monomer or oligomer (hereinafter referred to as a“hydrophilic monomer”) is selected from the group consisting of anoil-insoluble polar monomer or oil-insoluble polar oligomer, which issubstantially insoluble in a hydrophobic phase, and a polar monomer oroligomer (having both water-solubility and oil-solubility) other thanthe oil-insoluble monomer and oligomer. The content of the hydrophilicmonomer contained in the stock solution is usually from about 1 to about40% by weight, and preferably from about 2 to about 20% by weight, basedon the total weight of the stock solution.

Preferably, the water-insoluble polar monomer which can be used in asthe hydrophilic monomer, includes, for example, polyethylene oxideacrylate, polyethylene oxide diacrylate, polyethylene glycol acrylate,polyethylene glycol diacrylate, polyethylene urethane acrylate,polyethylene urethane diacrylate, acrylamide, sodium styrene sulfonate,sodium acrylate, sodium 2-acrylamide-2-methylpropane sulfonate, sodiummethacrylate, and a mixture thereof.

The hydrophilic monomer, which includes polyethylene oxide acrylate orpolyethylene oxide diacrylate, preferably, and includes polyethyleneoxide acrylate, particularly preferably, is effective for easily formingan adhesive hydrophilic polymer and enhancing the adhesion property ofthe adhesive containing the same.

The oil-insoluble monomers are preferably those which have thesolubility of about 0.5% by weight or less in oil (i.e. hydrophobicphase) and exhibit a distribution ratio (of the concentration in ahydrophobic phase to that in a hydrophobic phase) of about 0.005 orless. The solubility and concentration are values measured at a servicetemperature (usually about 25 to 35° C.) of the adhesive.

Various known polar monomers are so-called amphiphatic monomers whichexhibit a fixed solubility in both water and oil. Such an amphiphaticpolar monomer has the solubility of about 0.5% by weight or more in thehydrophobic phase and exhibits the distribution ratio (of theconcentration in the hydrophobic phase to that in the hydrophobic phase)of about 0.005 or more (at about 25 to 30° C., usually). These monomers,which can be dissolved in both phases, can be used in combination withthe oil-insoluble polar monomer. Useful monomers which can be used inthe present invention and partitioned between the hydrophilic phase andhydrophobic phase includes, for example, N-vinyl pyrroldione, N-vinylcaprolactam, (meth)acrylic acid, hydroxyethyl (meth)acrylate, itaconicacid, styrenesulfonic acid, N-substituted acrylamide, N,N-disubstitutedacrylamide, N,N-dimethylaminoethyl methacrylate,2-acrylamide-2-methylpropanesulfonic acid, and a mixture thereof.

Preferred partitionable monomers include acrylic acid, N-vinylpyrrolidone, N-vinyl caprolactam, N,N-dimethylacrylamide and a mixturethereof because of preferred effect capable of improving the physicalstrength (e.g. cohesive force of adhesive, etc.).

The hydrophilic phase of the conductive adhesive can further containvarious water-soluble additives. Various additives are appropriatelyselected to exhibit desired characteristics. For example, an electrolyteis added to exhibit the conductivity. To enhance the moisturizingeffect, a humectant is added. Examples of the other useful additivesinclude, but are not limited to, water-soluble crosslinking agents (e.g.triethylene glycol dimethacrylate, etc.), water-soluble plasticizers, pHadjustors, non-copolymerizable polar oligomers and mixtures thereof.

Preferably, the electrolyte includes, for example, potassium chloride,lithium chloride, sodium chloride or mixtures thereof. The content ofthe electrolyte is usually within a range from about 0.1 to about 10% byweight, preferably from about 0.5 to about 5% by weight, andparticularly preferably from about 0.8 to about 1.6% by weight, based onthe total weight of the adhesive. When the content is within the aboverange, the electrolyte dissolves in water contained in the hydrophilicphase to effectively function as an aqueous electrolyte solution.

The water-soluble plasticizer is preferably added as an additionalcomponent because the adhesion to the skin is enhanced. The content ofthe plasticizer is usually within a range from about 0.01 to about 10%by weight, and preferably from about 0.5 to about 5% by weight, based onthe total weight of the adhesive. When the content of the plasticizer istoo large or too small, there is a fear that a desired effect is notobtained. The water-soluble plasticizer includes, for example, thoseselected from the group consisting of poly(N-vinyl pyrrolidone),polyethylene glycol, poly(oxyethylene) alcohol, polyethylimine and amixture thereof, but is not limited thereto.

Humectant

As the humectant, for example, polyhydric alcohols such as glycerin,propylene glycol, etc. and amino acids can be used. Preferably, aminoacids having a higher moisturizing effect are used. Since the phasestructure is not broken when a large amount of the amino acids having ahigher moisturizing effect than that of glycerin and propylene glycolare used, there can be provided a biomedical electrode which is hardlydried even when stored under a low-moisture condition outside a sealedpouch. The biomedical electrode using the conductive adhesive containingamino acids can be effectively prevented from drying for a long periodof time in a sealed pouch resealed by using a sealing means afteropening the sealed pouch under a low-humidity condition. The content ofthe humectant is usually within a range from about 5 to about 50% byweight, preferably from about 10 to about 40% by weight, and morepreferably from about 15 to about 37% by weight, based on the totalweight of the adhesive.

Preferably, the humectant includes, for example, amino acids having ahigher moisturizing effect than that of glycerin such astrimethylbetaine, DL-pyrrolidonecarboxylic acid (PCA), sodiumDL-pyrrolidonecarboxylate, etc. The amino acids are particularlysuperior in effect of preventing drying of the adhesive.

Specific examples of trimethylbetaine which is useful as the humectantinclude, for example, Aquadew™ AN-10 which is commercially availablefrom Ajinomoto Co. Specific examples of sodium DL-pyrrolidonecarboxylateinclude “PCA soda” and “PCA” both of which are commercially availablefrom Ajinomoto Co.

Water-soluble Polymerization Initiator

The aqueous medium of the stock solution preferably contains awater-soluble free-radical photopolymerization initiator. Usefulphotopolymerization initiator is a water-soluble compound which acts asan initiator for polymerization reaction of a monomer or oligomer (thosecontaining a polymerizable surfactant described below) contained in thestock solution, thereby to form a free radical on exposure toelectromagnetic wave (usually ultraviolet light).

Useful water-soluble photopolymerization initiators includes, forexample, those selected from the group consisting of benzophenonesubstituted with an ionic group and/or a hydrophilic group; thoxanthonesubstituted with an ionic group and/or a hydrophilic group; and phenylketone such as 4substituted-(2-hydroxy-2-propyl)phenyl ketone (wherein4-substituent is an ionic group or a hydrophilic group). Preferably, theionic group or hydrophilic group include those selected from the groupconsisting of hydroxyl groups, carboxyl groups and carboxylate groups.

Useful water-soluble benzophenones include, for example, those selectedfrom 4-trimethylaminomethylbenzophenone hydrochloride, benzophenonesodium 4-methanesulfonate and benzophenone sodium 4methanesulfonate.Useful water-soluble thioxanthones include, for example, those selectedfrom the group consisting of3-(2-hydroxy-3-trirethylaminopropozy)thioxanthone hydrochloride,3-(3-trimethylaminopropozy)thioxanthone hydrochloride, thioxanthone3-(2-ethoxysulfonic acid) sodium salt and 3-(3-propoxysulfonic acid)sodium salt. Useful water-soluble phenyl ketones include, for example,those selected from the group consisting of phenyl ketone,(2-hydroxy-2-propyl)(phenyl-4-butanecarboxylate)ketone,4-(2-hydroxyethoxy)(phenyl-2-propyl)ketone and water-soluble saltsthereof. As far as the effect of the present invention is not adverselyaffected, water-soluble photopolymerization initiators are not limitedthereto. Particularly preferred water-soluble photopolymerizationinitiator is 1-(4-(2-hydroxy)-phenyl)-2-2-hydroxy-2-ethyl-propan-1-one).

The above-described water-soluble photopolymerization initiator can becontained in the aqueous medium in a different amount depending on thedesired effect. The content of the water-soluble photopolymerizationinitiator is usually within a range from about 0.05 to about 2% byweight, and preferably from about 0.1 to about 1% by weight, based onthe total weight of the stock solution.

Hydrophobic Phase

The hydrophobic phase contains a hydrophobic adhesive polymer orcopolymer. In the present specification, these polymer and copolymer aregenerically referred to as a “hydrophobic adhesive polymer.” Thehydrophobic adhesive polymer is usually a polymer of the second monomer(including a monomer mixture), comprising a hydrophobic free-radicallypolymerizable monomer and a free-radically polymerizable polar monomer.As the hydrophobic free-radically polymerizable monomer, for example,one or more kinds of them can be selected from alkyl(C1-C18) alcoholester of acrylic acid. It is selected so that the polymer produced fromthese monomers is an adhesive polymer. The glass transition temperature(Tg) of a useful hydrophobic free-radically polymerizable monomer(including a mixture) can be measured by a person with ordinary skillusing a known technique. Tg is usually 10° C. or less, preferably 0° C.or less, and particularly preferably −10° C. or less.

Preferred hydrophobic free-radically polymerizable monomers include, forexample, one or more kinds selected from isooctyl acrylate, 2-ethylhexylacrylate and n-butyl acrylate.

To produce the hydrophobic adhesive polymer, the hydrophobic secondmonomer contained in the stock solution optionally contains afree-radically polymerizable polar monomer, which is copolymerizablewith an alkyl acrylate, to control Tg of the adhesive polymer within apreferred range. Preferred polar monomer includes, for example, styrene,acrylonitrile and vinyl ester (e.g. vinyl acetate, vinyl propionate,vinyl neopentanoate, etc.).

The content of the hydrophobic second monomer is usually within a rangefrom about 5 to about 40% by weight, preferably from about 7 to about30% by weight, and particularly preferably from about 10 to about 20% byweight, based on the total weight of the stock solution so as to easilyimpart sufficient cohesive force and adhesion property to the conductiveadhesive as a polymerization product form the stock solution.

Oil-soluble Polymerization Initiator

To easily control the polymerization of the second monomer, the stocksolution preferably contains an oil-soluble free-radicalphotopolymerization initiator (photopolymerization initiator) and/or anoil-soluble free-radical thermopolymerization initiator (thermalpolymerization initiator).

Useful oil-soluble photopolymerization initiators are oil-solublecompounds which act as an initiator for polymerization reaction of amonomer or oligomer (those containing a polymerizable surfactantdescribed below) contained in the stock solution, thereby to form a freeradical on exposure to electromagnetic wave (usually ultraviolet light).

Useful oil-soluble photopolymerization initiators include, for example,those selected from the group consisting of (1) Michler's ketone andbenzophenone mixed in a weight ratio of about 1:4; (2) coumarin-basedphotopolymerization initiator described in U.S. Pat. No. 4,289,844; and(3) photopolymerization initiator containing dimethoxyphenylacetophenoneand/or diethoxyacetophenone as a base. A preferred photopolymerizationinitiator is 1-hydroxy-cyclohexyl phenyl ketone.

The oil-soluble initiator is an oil-soluble compound which is originallycontained in the hydrophobic phase of the stock solution and forms afree radical on polymerization operation (e.g. UV irradiation, etc.),said free radical capable of allowing to proceed the polymerization ofthe monomer.

Specific examples of the oil-soluble thermal polymerization initiatorwhich can be used in place of or in combination with thephotopolymerization initiator include, for example, azo compounds suchas “trade name: Vazo 64, 2,2′-azobis(isobutyronitrile)” or “trade name:Vazo 52, 2,2′-azobis(2,4-dimetyhylpentanenitrile)” (both of which arecommercially available from DuPont Co.). A peroxide such as benzylperoxide, lauroyl peroxide and a mixture thereof can also be used.Preferred oil-soluble thermal polymerization initiator is2,2′-azobis(isobutyronitrile).

The content of the water-soluble initiator (content of a mixture whenusing the photopolymerization initiator in combination with the thermalpolymerization initiator) is usually within a range from about 0.005 toabout 1% by weight, and preferably from about 0.01 to about 0.1% byweight, based on the total weight of the stock solution.

Hydrophobic Additive

The hydrophobic phase of the stock solution can optionally contain anadditional free-radically reactive additive such as oil-solublecrosslinking agent. Examples of useful crosslinking agent include thoseselected from the group consisting of divinylbenzene, alkyl (aboutC4-C8) diacrylate, 1,4-hexanediol diacrylate, 1,8-octanediol diacrylateand a mixture thereof, but are not limited thereto. Preferredcrosslinking agent is 1,6-hexanediol diacrylate. When the crosslinkingagent is added, the physical characteristics of the final polymer, forexample, cohesive force, insolubility to solvent, elasticity modulus,etc. are improved. The hydrophobic phase usually contains thecrosslinking agent in the amount within a range from about 0.01 to about10% by weight, preferably from about 0.02 to about 1% by weight, andparticularly preferably from about 0.05 to about 0.2% by weight, basedon the total weight of the stock solution.

Surfactant

The surfactant used preferably for preparing the conductive adhesive ofthe present invention may also be a reactive surfactant, which iscopolymerizable or not copolymerizable with the above monomer, and canbe used properly depending on the purpose. In the case of the reactivesurfactant, for example, the sensitivity of the conductive adhesive towater is lowered, thereby making it possible to easily prevent theadhesion property from drastically lowering due to an influence ofsweating.

The surfactant which can be usually used includes, for example, nonionicsurfactants, cationic surfactants, or anionic surfactants. The contentof the surfactant is usually within a range from about 0.01 to about 30%by weight, and preferably from about 5 to about 20% by weight, based onthe total weight of the adhesive.

The nonionic surfactant may be usually an organic aliphatic or alkylaromatic hydrophobic compound, and a condensation product of ahydrophilic alkylene oxide such as ethylene oxide. Almost all ofhydrophobic compounds having a carboxy, hydroxy, amido or amino groupwith liberation of hydrogen are condensed with ethylene oxide to form anonionic surfactant. A desired balance between hydrophobic andhydrophilic elements (hydrophilicity—lipophilicity balance or HLB) isattained by controlling the length of an ethylene oxide chain of thecodensate. HLB of the surfactant can be controlled by the size or kindof a hydrophilic (water-loving or polar) group and a lipophilic(oil-loving or non-polar) group of the surfactant. HLB of the nonionicsurfactant is usually from about 6 to about 19.

Useful noninic surfactants include, for example, those selected from thegroup consisting of non-copolymerizable nonionic surfactant,ethylenically-unsaturated copolymerizable nonionic surfactant and amixture thereof.

The anionic surfactant usually includes:

(a) a hydrophobic moiety selected from the group consisting of C6-C20alkyl group, alkylaryl group and alkenyl group, and

(b) a hydrophilic moiety comprising an anionic group selected from thegroup consisting of sulfate, sulfonate, phosphonate, polyoxyethylenesulfate, polyoxyethylene sulfonate, polyoxyethylene phosphonate andalkali metal and ammonium salts thereof, or tertiary amino salt group ofthese anionic groups.

A copolymerizable surfactant comprising C2-C18 alkenyl polyoxypropyleneor C2-C18 polyoxybutylene as the hydrophobic moiety, an anionic group ofpolyoxyethylene sulfate as the hydrophilic moiety, and anethylenically-unsaturated double bond is also useful.

To obtain a more stable macro- or micro-emulsion, anethylenically-unsaturated polymerizable anionic surfactant is preferred.Specific examples of the copolymerizable anionic surfactant includeMazonm™ SAM 211 which is commercially available from PPG Industries Inc.and Adekareasor™ SE-10N (product No.): ammonia salt ofα-sulfo-χ{1-nonylphenoxymethyl-2-(2-propenyloxy)ethoxy}-poly(oxy-1,2-ethanediyl) manufactured by Assahi Denka Kogyo Co.

The non-reactive surfactant is preferably sodium polyoxyethylenealkyl(C10-C16) ether sulfates such as Emale™ E-27C, Emale™ E-70C, etc.,which are commercially available from Kao Corp.

As the cationic surfactant, for example, there can be used quaternaryammonium salts wherein at least one higher molecular weight groups(having 6 or more carbon atoms) and two or more lower molecular weightgroups (having 1 to 5 carbon atoms) are linked to a common nitrogen atomto produce a cation, resulting in electrical balance. In this case, theanion includes those selected from the group consisting of halide (e.g.bromide, chloride, etc.), acetate, nitrate and lower alkosulfate (e.g.methosulfate, etc.), but are not limited thereto.

Precursor syrup

The above macro- or micro-emulsion structure is not formed easily and ispreferably formed in the following procedure.

First, components other than the crosslinking agent and water-solublepolymerization initiator among the raw material components are chargedin a UV reactor or thermal reactor, and then mixed with purging anitrogen gas in the reactor. When using the photopolymerizationinitiator, the raw material mixture in the reactor is polymerized by UVirradiation. The UV intensity of a UV lamp for UV irradiation is from0.1 to 10 mW/cm². When using the thermal polymerization initiator, themixture is polymerized by heating.

Usually, the polymerization treatment is conducted with stirring and thepolymerization treatment is conducted until the domain structurecontaining the hydrophobic adhesive polymer is fixed, that is,stabilized. The end point of the formation of the precursor syrupsolution is decided by the viscosity of the emulsion containing thedomain phase. Preferred viscosity is from 100 to 4000 cps.

On the other hand, the respective components are preferably used in theamount within the above range. It is particularly preferred that theamount of water, and kind and amount of the humectant are decided, andthen the amount of other components is decided so as to obtain desiredcharacteristics. Water and humectant are important factors to controlthe initial amount of water contained and retained, and the amount ofwater contained in the adhesive is a principal factor which exerts agreat influence on the conductivity and adhesion property.

The step of deciding the composition is now described with reference toa specific example. For example, 1.5 parts by weight of an electrolyteis dissolved in 36 parts by weight of water to prepare an electrolytesolution. Then, 22 parts by weight of the above-described humectant ofamino acids is added as the humectant so as to attain a water retention(percentage) of about 10% under the conditions of 20° C.-20% RH(relative humidity).

The formulation of the respective components is decided so that thetotal amount of the hydrophobic monomer, hydrophilic monomer andwater-soluble plasticizer becomes 40 parts by weight based on 59.5 partsby weight of the above components. This formulation ratio is decided sothat the physical properties such as adhesion property, flexibility,etc. are within the desired range. When using the amphiphatic monomer,the amount may be controlled to 40 parts including the amount of themonomer. Furthermore, 0.05 parts by weight of an oil-solublephotopolymerization initiator is dissolved to obtain a mixed solution ofthe total amount of 100 parts by weight.

The above mixed solution is a white turbid emulsion during the stirring,but is separated when the stirring is terminated. To form a more stableemulsion, a predetermined amount of the surfactant is added to about 100parts by weight of the above mixed solution so that the resultingsolution is a uniform opaque liquid during the stirring and is notseparated soon even when the stirring is terminated. The amount of thesurfactant varies depending on the kind of the surfactant andincorporation of components other than the surfactant, but is preferablyfrom 5 to 20 parts by weight.

In such way, the above comparatively stable emulsion is obtained as thestock solution. Subsequently, this stock solution is irradiated with UVwith stirring to obtain a precursor syrup solution as an adhesiveprecursor of a stable emulsion. Then, 0.1 parts by weight of acrosslinking agent and 0.5 parts by weight of a water-solublephotopolymerization initiator are added to the resulting syrup solution,and the mixture is irradiated with UV to obtain an adhesive compositionof the present invention as a final product. The amount of the aboverespective components can be controlled to satisfy the requirecharacteristics by repeating tests.

Formation of Conductive Adhesive Layer

After a macro- or micro-emulsion was once formed by the above method forconversion into a syrup, a final product, i.e. a conductive adhesive ofthe present invention can be obtained by polymerization of the emulsionin accordance with a known procedure.

For example, the precursor syrup solution obtained as described above iscoated on a proper substrate by using a conventional means such asroller coating, dip coating, knife coating, extrusion coating, etc. toform a layer of the precursor-syrup solution. At this time, awater-soluble polymerization initiator, and a crosslinking agent to beadded optionally are added to the precursor syrup solution.

Then, the formed layer of the precursor syrup solution (hereinafterreferred to as a syrup layer) is polymerized in an inert atmosphere (ina state free from oxygen, e.g. under a nitrogen atmosphere) to form anadhesive layer of a conductive adhesive. For example, UV irradiation isconducted under anaerobic conditions by laying a plastic film, which issubstantially permeable to ultraviolet light but impermeable to oxygen,on the syrup layer, thereby to cause the polymerization reaction. UVirradiation can be usually conducted through the plastic film, using afluorescent type ultraviolet lamp which emits ultraviolet light in awavelength range absorbed by a UV photopolymerization initiator. Theplastic film used herein is preferably a polyester film whose surface tobe made in contact with the syrup layer is a silicone release surface.

UV irradiation can be preferably conducted by using a plurality ofcommercially available different lamps. Examples of the lamp includecombination of low-pressure mercury lamp or medium-pressure mercury lampand low-intensity fluorescent lamp. In this case, each lamp has adifferent emission spectrum and emits light having a maximum intensitywithin a range from 280 to 400 nm. Usually, a commercially availableblack lamp, which has a maximum of the intensity at about 350 nm andshows 90% of the intensity distribution within a range from 300 to 400nm, is used.

The total dose on UV irradiation is usually from about 200 to 5000millijoules (mJ)/cm². Usually, the efficiency and rate of polymerizationis decided by the relation between emission properties of an irradiationlight source and absorption characteristics of the UVphotopolymerization initiator.

Preferred photopolymerization process is a process of continuouslyexposing the syrup layer to electromagnetic wave of about 350 nm forenough time to provide the dose of about 2000 mJ/cm².

The thickness of the conductive adhesive layer varies depending on thestate on use, but is from about 0.1 to about 4 mm, and preferably fromabout 0.5 to about 2 mm. When the thickness of the conductive layer istoo small, the anti-drying performance is likely to be lowered. On theother hand, when the thickness is too large, the volume of wholebiomedical electrode becomes large, resulting in poor handling.

UV irradiation can be conducted from one surface of the syrup layer, butis preferably conducted from both surfaces. In the syrup layer having athickness of 0.8 mm or more, the syrup is white or opaque and,therefore, the light scattering effect in the emulsion can not beneglected. Accordingly, irradiation from both surfaces is preferred sothat both surfaces of the adhesive layer have the same properties.

Biomedical Electrode

A biomedical electrode comprising an adhesive layer of the conductiveadhesive is particularly useful for diagnosis (including monitoring) andtreatment in the medical field. In a basic form, the biomedicalelectrode comprises an electrode terminal which is made contact with theskin of mammals (including human) as a patient or subject, therebymaking it possible to form mutual electrical communication between aconductive adhesive layer and an electrical diagnostic, therapeutic orelectrosurgical equipment.

FIG. 1 and FIG. 2 each shows one preferred embodiment of the biomedicalelectrode according to the present invention. The biomedical electrodeshown in the drawing is disposable and can be applied to the skin of thepatient on use so as to obtain an electrocardiogram (ECC or EKG) or toafford a transcutaneous electrical nerve stimulation (TENS).

A biomedical electrode 10 shown in the drawing comprises a field of aconductive adhesive layer to be made in contact with the patient's skinon removal of a release liner 12 for protection of the adhesive layer(hereinafter referred to as a conductive field) 14, an electricalconnection means 16 including a conductive film 26 having a conductiveinterface portion, which is made contact with the conductive field 14,and a tab portion 20 for mechanical or electrical connection with anelectrical instrumentation (not shown), which extends beyond theconductive field 14. The electrical connection means 16 finctions as anelectrode terminal. The electrical connection means 16 includes asubstrate 22, and a conductor film 26 coated on a principal surface madein contact with at least conductive field 14 of the substrate.

The typical conductor film 26 includes a substance having a thickness ofabout 0.05 to 0.2 mm, e.g. strip of a polyester film, and has asilver/silver chloride coating layer having a thickness of about 2.5 to12 μm, (preferably about 5 μm) thereon. Specific examples of theconductor film 26 include coating layer of conductive ink, i.e.silver/silver chloride ink R-300 (product No.) which is commerciallyavailable from Ercon Co. The electrical connection means 16 can beobtained, for example, by providing this coating layer on a principalsurface of a polyester film substrate which is commercially availablefrom 3M Co. under the trade name of “Scotchpar™” or which iscommercially available from ICI Co. under the trade name of “Melinex™”505-300, 329 or 339.

The electrical connection means 16 of the biomedical electrode for TENScomprises a non-woven fabric such as fabric of polyester/cellulosefibers which is commercially available from Lydal Co. under the tradename of “Mannlweb™”, and has a conductor film of a carbon ink layerwhich is commercially available from Acheson Coiloids Co. under thetrade name of Carbon Ink SS243 63 (product No.) on the principalsurface.

To enhance mechanical contact between an electrode clip (not shown) andthe conductor film 16, an adhesive tape with a substrate of polyethylenecan be applied to the tab portion 20 located on the surface opposite theprincipal surface having a conductive coating 26. As the adhesive tape,a surgical tape, which is commercially available from 3M Co. under thetrade name of “Blenderm™”, can be used.

A conductor comprising a non-conductive flexible polymer film and aconductive layer having a multi-layer structure as disclosed inInternational Publication WO9741568 can be used selectively as the aboveelectrical connection means. This conductor has a conductive layercomposed of a base conductive layer having small porosity, which isformed by coating conductive ink containing a hydrophobic polymerbinder, silver particles and carbon particles on a polyester film, and atop conductor layer having large porosity, which is formed by coatingconductive ink containing a polymer binder, Ag/AgCl particles and carbonparticles on the base conductive layer.

A biomedical electrode having another structure is shown in FIG. 3 as across-sectional view. A biomedical electrode 80 shown in the drawingincludes a non-conductive coating 82 having an opening 83 covered by asnap 84 through which an electrode terminal including an eyelet 85protrudes. The snap 84 is fixed to coat the eyelet 85, thereby to giveelectrical connection with an electrical instrumentation. The eyelet 85is electrically connected with a conductive adhesive layer 86, and theadhesive layer 86 of the conductive adhesive of the present invention isperfectly coated with the snap 84 and the non-conductive coating 82.

A release liner 88 is used for protecting the adhesive layer 86 beforeuse. The non-conductive coating 82 is usually made of an insulatingsubstance. The eyelet 85 is, for example, an eyelet made of a plasticmetal plate. Such an eyelet is, for example, an ABS plastic eyelet madeof a silver plate or a chloride, and is commercially available fromMicron Products Co. The snap 84 is, for example, a metal snap. Such asnap is, for example, a stainless eyelet which is commercially availablefrom Eyelet for Industry of Thomson under the product No. 304.

Other examples of biomedical electrodes, which can use the conductiveadhesive of the present invention, include electrodes disclosed in thefollowing U.S. Pat. Nos. 4,524,087, 4,539,996, 4,554,924, 4,848,353,4,846,185, 4,771,713, 4,715,382, 5,012,810 and 5,133,356. The method ofproducing these electrodes is not disclosed in each specification, butthe conductive adhesive of the present invention can be replaced by theconductive adhesive disclosed herein.

Packaging of Biomedical Electrode

On actual use of the biomedical electrode, a fixed number (e.g. 3, 5 or10) of biomedical electrodes required for single inspection are appliedto one release liner and a fixed number (e.g. 1, 5 or 10) of releaseliners with biomedical electrodes, on which the required numbers ofbiomedical electrodes are applied, are put in a sealed pouch made froman aluminum foil and then stored and taken out immediately before use.

In case where the biomedical electrode stored in the sealed pouch isonce taken out, put in the sealed pouch again and allowed to stand undera low-humidity condition, it is difficult to store the release linerwith the biomedical electrode in a conventional sealed pouch for a longperiod of time. It is difficult to completely reseal a conventionalsealed pouch. Accordingly, a sealed pouch provided with a simple sealingmeans (e.g. so-called zip lock fastener, etc.) for simply resealingafter opening is preferably used to maintain predeterminedcharacteristics (e.g. conductivity, etc.) under a low-humidity conditioneven after opening of the sealed pouch.

In the biomedical electrode according to the present invention, since aconductive adhesive having a high moisturizing effect in which drying iseffectively prevented is used, a liner with a plurality of electrodes isstored in a sealed pouch provided with a simple sealing means afteropening and a part of the biomedical electrode can be used in severalportions, if necessary.

EXAMPLES Example 1 (1) Preparation of Precursor Syrup Solution(Hereinafter Referred to as a “Syrup Number N1”) Composition ofPrecursor Syrup Solution N1

TABLE 1 (N1) Parts by % by Raw materials weight weight Monomers UV(Oil)Irg. 184 0.05 0.04 AA AA 14 12.10 IOA IOA 14 12.10 Surfactant SE-10N17.4 15.04 Aqueous MPEG550MA 8 6.91 monomer Plasticizer PEG300 3 2.59Salt solution Humectant Aquadew 22 19.01 4% KCl 40% KCL 37.25 32.20Total 115.7 100.000

The precursor syrup solution N1 of this example is a micro-emulsionhaving the above-described composition. The solution was prepared bystirring a mixed solution of raw materials described in Table 1 withsubjecting to UV irradiation in a UV reaction vessel equipped with aglass container, a propeller coated with Teflon™, purge line of anitrogen gas, a vessel cover and a UV lamp. The procedure is generallydescribed below.

The glass container of the UV reaction vessel was filled with thefollowing raw materials:

“Irgacure 184™, 1-hydroxy-cyclohexyl phenyl ketone” manufactured by CibaGeigy Co. as an oil-soluble UV photopolymerization initiator (UV),

isooctyl acrylate (IOA) as a second monomer capable of producing ahydrophobic adhesive polymer,

methoxypolyethylene glycol 550 monoacrylate (MPEG550MA, manufactured bySatomer Co.) and acrylic acid (AA) as a first monomer capable ofproducing an adhesive hydrophilic monomer, and

polyethylene glycol 300 (PEG300) as a plasticizer, thereby to obtain amixed solution.

After stirring at about 100 rpm until Irgacure 184 was dissolved, anaqueous 4% KCl solution and a reactive surfactant Adekareasoap™ SE-10N(product No.) manufactured by Asahi Denka Kogyo Co. were added to theresulting mixed solution. After the dissolution of Adekareasoap wasvisually confirmed, a humectant Aquadew™ AN-100 (product No.)manufactured by Ajinomoto Co. was further added.

After the dissolution of Aquadew was visually confirmed, a nitrogen gaswas purged in the glass container with stirring at 80 rpm. Using a UVlamp controlled so that a measured intensity at the surface of the glasscontainer becomes 2 mW/cm², UV irradiation was conducted for about 40seconds until bubbles of the gas evolve slowly. As a result of a seriesof operations, the appearance of the mixed solution was changed toopaque from transparent. This change means that a precursor syrupsolution N1 for preparation of a desired adhesive was formed in thisexample.

Then, the average diameter of a domain phase of the resulting precursorsyrup solution N1 was measured by a laser scattered particles analyzerLS230 (model No.) manufactured by Coulter Inc. As a result, it was 0.2μm. The viscosity of the precursor syrup solution N1 was measured by aB-M type viscometer. As a result, it was 160 cps at 25° C.

(2) Preparation of Two Kinds of Coating Syrup Solutions (HereinafterReferred to as “Syrup No. N1-a” and “syrup No. N1-b”)

Composition of Coating Syrup Solution N1-a:

TABLE 2 (N1-a) Parts by % by Raw materials weight weight PrecursorPrecursor syrup solution 115.7 99.53 N-1 UV photoinitiator Irg. 2959 0.50.43 Crosslinking agent TEGDMA 0.05 0.04 Total 116.25 100.00

Water content: 30.8%

Composition of Coating syrup solution N1-b:

TABLE 3 (N1-b) Parts by % by Raw materials weight weight PrecursorPrecursor syrup solution 115.7 99.48 N-1 UV photoinitiator Irg. 2959 0.50.43 Crosslinking agent TEGDMA 0.1 0.09 Total 116.3 100.00

Water content: 30.8%

To the precursor syrup solution N1 thus prepared, raw materialsdescribed in Table 2 or Table 3 were added, followed by sufficientmixing. “Irgacure™-2952”{1-(4-(2-hydroxyethoxy)-phenyl)-2-hydroxy-2-methyl-1-propan-1-one }manufactured by Ciba Geigy Co. as a water-soluble UV photopolymerizationinitiator and triethylene glycol dimethacrylate (TEGDMA) manufactured byTokyo Chem. Ind. as a crosslinking agent were added, respectively. As aresult, coating syrup solutions N1-a and N1-b each having the abovecomposition were obtained. Each coating syrup solution was also anopaque liquid having a mini-emulsion structure.

(3) Preparation of Conductive Adhesive (Adhesive Sheet)

Using a UV coater equipped with a low-pressure TV lamp at top and bottompositions, the coating and curing (crosslinking) of the coating syrupwere conducted. First, the coating syrup solutions N1-a and N1-bprepared as described above were coated on a KC tissue Scrim™manufactured by Kimberly Clark Co. placed on a white double-coated linerin a coating thickness of 1 mm using a knife coater. On coating of eachsyrup solution, a transparent liner was laminated to contact the surfaceof the syrup solution with the silicone release surface, and then thesyrup solution was coated by placing a knife on the transparent liner.

A power source of top and bottom UV lamps was controlled so as to obtainthe same intensity through both liners (white double-coated liner andtransparent liner) and the layer of the coating syrup interposed betweenthe liners was cured. The energy integrated on the surface of the coatedlayer of the syrup solution was 1050 mJ/cm² per one surface.

After the completion of the curing treatment due to UV irradiation, aconductive adhesive interposed between both liners of this example wasobtained. That is, an adhesive sheet comprising a reinforcing substrateof the above scrim and an adhesive layer of a conductive adhesive withwhich the reinforcing substrate is impregnated of this example wasobtained. In this state, the obtained one has a form of an adhesivesheet with double-coated liner wherein both adhesive surfaces areprotected with the double-coated liner. This adhesive sheet with thedouble-coated liner is adhered to a backing film after peeling off oneliner, thereby making it possible to use as a conductive adhesive filmhaving a three-layer structure of backing film/adhesive sheet/liner.

After the adhesive sheet with the double-coated liner prepared asdescribed above was stored in a vinyl bag for one day, the thickness ofthe adhesive sheet was measured. As a result, it was 0.8 to 0.9 mm. Theresidual monomer of the adhesive sheet was analyzed by a gaschromatography (GC) and a liquid chromatography (LC). As a result, theresidual monomer was not detected. The detection limit was 10 ppm.

(4) Production of Biomedical Electrode

The adhesive sheet of this example was incorporated into a stud typeelectrode described previously with reference to FIG. 3. Accordingly,this biomedical electrode has a structure of an adhesive sheet (withsingle-coated liner), a backing of polyethylene and polypropylene, acarbon stud and a Ag/AgCl black eyelet. The adhesive sheet wasincorporated by peeling off one liner from the adhesive sheet with theboth-coated liner made as described above, and adhering to a backing.

The biomedical electrode made as described above was stored in a sealedpouch made of an aluminum foil, and also stored in a sealed pouch with azip lock fastener for resealing the opened pouch.

(5) Characteristics Evaluation Test

The characteristics of the biomedical electrode made as described abovewere evaluated in accordance with the following criteria and procedures.

Lifetime of Electrode

A target of the lifetime of the biomedical electrode may be two years incase where the biomedical electrode is stored in a sealed pouch at roomtemperature. The present inventors have recognized based on experiencethat the electrode has the lifetime of two years at room temperature of20 to 25° C. if the electrode aged at 57° C. for 10 weeks or aged at 66°C. for 6 weeks meets the electrical characteristics defined in thestandard of AAMI (Association for the Advancement of MedicalInstrument). Accordingly, in the characteristics evaluation test of thisexample, it was judged whether or not the biomedical electrode can havefixed electrical characteristics and adhesion characteristics even afteraging. Furthermore, the adhesion strength to the skin of human as atypical subject was also measured at initial stage and after aging(after 3 weeks and 6 weeks).

Resistance to Drying

A target of the resistance to drying of the biomedical electrode is thatthe electrode can sufficiently maintain AAMI characteristics duringstorage under the conditions of 20° C.-20% RH (relative humidity)outside a sealed pouch for 30 days. Accordingly, in the characteristicsevaluation test of this example, it was judged whether or not theelectrode stored outside the sealed pouch and the electrode, which wasput in an opened sealed pouch and then resealed by a zip lock fastener,can have have fixed electrical characteristics and adhesioncharacteristics even after aging. Furthermore, the adhesion strength tothe skin of human as a typical subject was also measured at initialstage and after aging (after 3 weeks and 6 weeks).

The AAMI characteristics refer to proper performances that aredetermined with respect to a biomedical electrode used in an EGGdisposable electrode by AAMI using the following criteria and testprocedures. The test procedure and standard with respect to the minimumcriteria are composed of the following four items.

(1) DC offset potential 100 mV or less (2) AC impedance 2000 Ω or less(3) Offset potential after 5 seconds 100 mV or less have passed sincedefibrillation (4) Recovery speed after 5 seconds 0-1.0 mV/s have passedsince defibrillation

(change in residual polarization potential after 5 seconds have passedsince 4 charging/discharging)

Accordingly, in the characteristics evaluation test of this example, theevaluation tests for four items described above were carried out byusing an ECC electrode tester Xtrateck EF-68ATM (manufactured byXtrateck Co.). At initial stage and after aging or drying, therespective biomedical electrodes was connected back to back(adhesive-adhesive) with each other to make a pair of test electrodes,and then the measurement was carried out in accordance with a manualdescribed in the tester.

Subsequently, the adhesion test (skin adhesion test) of the adhesivesheet of this example was carried out in the following procedure.

First, the above single-coated liner was laid on a polyester film tomake an adhesive film. The resulting film was cut into pieces of2.54×7.6 cm in size to obtain a strip-like test piece. The resultingtest piece was placed on the subject's chest and applied uniformly byrolling a roller of 1 kg. Using a mechanical peeling device referred toas an adhesion tester, the test piece was peeled off immediately beforeapplication. The peeling condition of the strip was as follows: speed of12 inch/min and a direction of 180 degree. Each adhesion strength(adhesion force) was recorded by the number of gram per 2.54 cm (1 inch)[g/2.54 cm].

As a result of a series of characteristics evaluation tests as describedabove, the measurement results described in Table 4 to Table 9 wereobtained.

TABLE 4 Aging stability of a stud type biomedical electrode derived froma coating syrup solution N1-a in a sealed pouch at 66° C. AAMI Testsspecification Initial 3 weeks 6 weeks DC offset potential 100 mV or less 0.1 mV −0.2 mV −0.3 mV AC impedance 2000 Ω or less 198 Ω 203 Ω 188 ΩOffset potential after 100 mV or less 10.2 mV 11.2 mV 12.1 mV 5 secondshave passed since defibrillation Recovery speed 0 to −1.0 mV/s −0.2 mV/s−0.3 mV/s −0.3 mV/s after 5 seconds have passed since defibrillationAdhesion strength 100 to 400 g 215 g 290 g 290 g of stud type per eachper each per each per each electrode electrode electrode electrodeelectrode

TABLE 5 Aging stability of a stud type biomedical electrode derived froma coating syrup solution N1-b in a sealed pouch at 66° C. AAMI Testsspecification Initial 3 weeks 6 weeks DC offset potential 100 mV or less−0.2 mV −0.3 mV  0.5 mV AC impedance 2000 Ω or less 251 Ω 213 Ω 183 ΩOffset potential after 100 mV or less 11.1 mV 11.2 mV 12.7 mV 5 secondshave passed since defibrillation Recovery speed after 5 seconds have 0to −1.0 mV/s −0.2 mV/s −0.3 mV/s −0.3 mV/s passed since defibrillationAdhesion strength 200 to 400 g 180 g 255 g 320 g of stud type per eachper per per electrode electrode each each each electrode electrodeelectrode

TABLE 6 Resistance to drying of a stud type biomedical electrode derivedfrom a coating syrup solution N1-a under 20° C.-20% RH outside a sealedpouch AAMI Tests specification Initial 10 days 20 days 30 days DC offsetpotential 100 mV or less 0.1 mV 0.0 mV −0.5 mV −0.4 mV AC impedance 2000Ω or less 198 Ω 427 Ω 1009 Ω 1756 Ω Offset potential after 100 mV orless 10.2 mV 11.8 mV 13.7 mV 16.7 mV 5 seconds have passed sincedefibrillation Recovery speed after 0 to −1.0 mV/s −0.2 mV/s −0.3 mV/s−0.3 mV/s −0.5 mV/s 5 seconds have passed since defibrillation Adhesionstrength of stud 100 to 400 g 215 g per each 360 g per each 375 g pereach 475 g per each type electrode per each electrode electrodeelectrode electrode electrode

TABLE 7 Resistance to drying of a stud type biomedical electrode derivedfrom a coating syrup solution N1-b under 20° C.-20% RH outside a sealedpouch AAMI Tests specification Initial 10 days 20 days 30 days DC offsetpotential 100 mV or less −0.2 mV 1.5 mV 0.2 mV −0.1 mV AC impedance 2000Ω or less 251 Ω 445 Ω 831 Ω 1878 Ω Offset potential after 100 mV or less11.1 mV 11.7 mV 13.3 mV 16.1 mV 5 seconds have passed sincedefibrillation Recovery speed after 0 to −1.0 mV/s −0.2 mV/s −0.3 mV/s−0.3 mV/s −0.4 mV/s 5 seconds have passed since defibrillation Adhesionstrength of stud 100 to 400 g 215 g per each 415 g per each 440 g pereach 460 g per each type electrode per each electrode electrodeelectrode electrode

TABLE 8 Resistance to drying of a stud type biomedical electrode derivedfrom a coating syrup solution N1-a under 20° C.-20% RH outside a sealedpouch (resealed by zip lock fastener) AAMI Tests specification Initial20 days 30 days DC offset potential 100 mV or less 0.1 mV 0.1 mV −0.4 mVAC impedance 2000 Ω or less 198 Ω 303 Ω 247 Ω Offset potential after 100mV or less 10.2 mV 10.8 mV 10.1 mV 5 seconds have passed sincedefibrillation Recovery speed after 0 to −1.0 mV/s −0.2 mV/s −0.2 mV/s−0.2 mV/s 5 seconds have passed since defibrillation Adhesion strengthof 100 to 400 g per 215 g per each 235 g per each 200 g per each studtype electrode each electrode electrode electrode electrode

TABLE 9 Resistance to drying of a stud type biomedical electrode derivedfrom a coating syrup solution N1-b under 20° C.-20% RH outside a sealedpouch (resealed by zip lock fastener) AAMI Tests specification Initial20 days 30 days DC offset potential 100 mV or less −0.2 mV −0.2 mV −0.3mV AC impedance 2000 Ω or less 251 Ω 202 Ω 257 Ω Offset potential after100 mV or less 11.1 mV 10.3 mV 10.6 mV 5 seconds have passed sincedefibrillation Recovery speed after 0 to −1.0 mV/s −0.2 mV/s −0.2 mV/s−0.2 mV/s 5 seconds have passed since defibrillation Adhesion strengthof 100 to 400 g per 180 g per each 180 g per each 170 g per each studtype electrode each electrode electrode electrode electrode

As is apparent from the evaluation results described in Table 4 to Table9 described above, the biomedical electrode of this example was notdried even when stored under a low-humidity condition of 20° C.-20% RHoutside a sealed pouch and sufficiently met the AAMI characteristics.However, when the biomedical electrode was stored under a low-humiditycondition of 20° C.-20% RH outside a sealed pouch for 30 days, the ACimpedance exceeded 1000 Ω and the adhesion strength was slightlyincreased. As a counter measure, when the biomedical electrode wasstored in a sealed pouch with a zip lock fastener for resealing theopened sealed pouch, the AAMI characteristics and adhesion strength canbe maintained at almost the same level for 30 days even when thebiomedical electrode is stored under the low-humidity condition of 20°C.-20% RH outside the sealed pouch.

The conductive adhesive as a constituent material of the biomedicalelectrode is derived from the coating syrup solution N1-a or N1-b. Ithas been found that the adhesion level of the resulting biomedicalelectrode can be controlled by utilizing a difference between these twokinds of syrup solutions. That is, the difference between both syrupsolutions lies in amount of a crosslinking agent to be added as isapparent from a comparison between Table 2 and Table 3. In case where alarge amount of the crosslinking agent is added, the adhesive becomesrigid and, therefore, the adhesion level can be reduced.

Comparative Example

In this example, for the comparison purpose, an adhesive consisting of amicroemulsion containing a hydrophobic adhesive phase and a hydrophilicconductive phase was prepared. The composition of the adhesive wassimilar to that of Example 1.

A mixture obtained upon mixing of the starting materials except for theabove-described humectant was a transparent liquid. This means that asize of the hydrophobic phase and the hydrophilic phase is smaller thanthe wavelength of visible light. That is, this is because a domain phaseof the emulsion has an average diameter of less than 0.02 μm.

On the other hand, when the humectant used in Example 1 was added to theabove-prepared transparent liquid, the hydrophilic phase and thehydrophobic phase were separated as upper and lower layers in thecontainer. This means that in the microemulsion containing a domainphase of the above-described size, use of an amino acid humectant isdifficult to maintain a stable phase structure as in Example 1.

Example 2

To obtain the better resistance to drying (drying prevention effect) asdescribed above, a large amount of the humectant is preferably added aspossible. In this example, the procedure described in Example 1 wasrepeated, except that the coating syrup solution N1-a was prepared afterthe amount of the humectant (the same as that used in Example 1) in theprecursor syrup solution N1 of Example 1 was changed, as shown in Table10-1 to Table 10-7, so as to examine an influence of an increase inamount of the humectant on an improvement in resistance to drying. Theprecursor syrup solution prepared in this example is referred to as a“syrup number N2” for distinction from that of Example 1.

TABLE 10-1 (N2-0) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.053 AA AA 14 14.862 IOA IOA 14 14.862Surfactant SE-10N 17.4 18.471 Aqueous MPEG550MA 8 8.493 monomerPlasticizer PEG300 3 3.185 Salt solution Humectant Aquadew 0 0.000 4%KCl 4% KCl 37.25 39.544 UV initiator Irg. 2959 0.5 0.531 Total 94.2100.000

TABLE 10-2 (N2-5) Parts by % by Raw Materials weight weight Monomers UVinitiator Irg. 184 0.05 0.050 AA AA 14 14.113 IOA IOA 14 14.113Surfactant SE-10N 17.4 17.540 Aqueous monomer MPEG550MA 8 8.065Plasticizer PEG300 3 3.024 Salt solution Humectant Aquadew 5 5.040 4%KCl 4% KCl 37.25 37.550 UV initiator Irg. 2959 0.5 0.504 Total 99.2100.000

TABLE 10-3 (N2-10) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.048 AA AA 14 13.436 IOA IOA 14 13.436Surfactant SE-10N 17.4 16.699 Aqueous monomer MPEG550MA 8 7.678Plasticizer PEG300 3 2.879 Salt solution Humectant Aquadew 10 9.597 4%KCl 4% KCl 37.25 35.749 UV initiator Irg. 2959 0.5 0.480 Total 104.2100.000

TABLE 10-4 (N2-15) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.046 AA AA 14 12.821 IOA IOA 14 12.821Surfactant SE-10N 17.4 15.934 Aqueous monomer MPEG550MA 8 7.326Plasticizer PEG300 3 2.747 Salt solution Humectant Aquadew 15 13.736 4%KCl 4% KCl 37.25 34.112 UV initiator Irg. 2959 0.5 0.458 Total 109.2100.000

TABLE 10-5 (N2-18) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.045 AA AA 14 12.478 IOA IOA 14 12.478Surfactant SE-10N 17.4 15.508 Aqueous monomer MPEG550MA 8 7.130Plasticizer PEG300 3 2.674 Salt solution Humectant Aquadew 18 16.043 4%KCl 4% KCl 37.25 33.200 UV initiator Irg. 2959 0.5 0.446 Total 112.2100.000

TABLE 10-6 (N2-22) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.043 AA AA 14 12.048 IOA IOA 14 12.048Surfactant SE-10N 17.4 14.974 Aqueous monomer MPEG550MA 8 6.885Plasticizer PEG300 3 2.582 Salt solution Humectant Aquadew 22 18.933 4%KCl 4% KCl 37.25 32.057 UV initiator Irg. 2959 0.5 0.430 Total 116.2100.000

TABLE 10-7 (N2-40) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.036 AA AA 14 10.057 IOA IOA 14 10.057Surfactant SE-10N 17.4 12.500 Aqueous monomer MPEG550MA 8 5.747Plasticizer PEG300 3 2.155 Salt solution Humectant Aquadew 0 0.000 4%KCl 4% KCl 45 32.328 UV initiator Irg. 2959 37.25 26.760 UV (Oil) Irg.2959 0.5 0.359 Total 139.2 100.000

In the above-described various precursor syrup solutions, since thehumectant is not added to the precursor syrup solution N2-0 forreference described in Table 10-1, this syrup solution was a transparentand stable micro-emulsion. In precursor syrup solutions N2-5, N2-10,N2-15, N2-18, N2-22 and N2-40 shown in the following other amount of thehumectant is changed as described in the tables.

The procedure described in Example 1 was repeated. As a result, each ofthe precursor syrup solutions N2-5, N2-10 and N2-15 wherein the amountof the humectant is 15% by weight (13.7% by weight) or less was a stableemulsion before conversion into a syrup (polymerization of hydrophobicmonomer), while a mixed solution of precursor syrup N2-18, N2-22 andN249 wherein the humectant was added in the amount of 18 parts by weight(16.0% by weight) or more was separated into two phases. That is, it hasbeen found that in the absence of the syrup formation process, itdifficult to prepare a stable emulsion adhesive wherein a comparativelylarge amount of the humectant of amino acids was added.

To obtain the resistance to drying in a sufficiently high level withrespect to the resistance to drying, the addition of 20% by weight of anamino acid humectant is desired. According to the present invention, astable mini-emulsion containing a comparatively large amount of aminoacids can be prepared by employing the above-described method forconversion into syrup.

The average domain diameter of the hydrophobic phase measured by a laserscattered particles analyzer of the precursor syrup solution N2-22prepared by the method for conversion into syrup in accordance with thepresent invention was from 0.1 to 0.8 μm. Accordingly, both of theprecursor syrup and conductive adhesive prepared by subjecting theprecursor syrup solution to a polymerization treatment were opaque. Theconductive wherein the humectant of this example was added had goodelectrical characteristics and resistance to drying in the level suitedfor use in the biomedical electrode.

Example 3

To obtain the better resistance to drying in a particularly suitablelevel as described above, about 20% by weight or more of the humectantis preferably added. In the precursor syrup solution for formation of anadhesive with these features, the size of the domain can be controlledby the composition and formulation. In light of the stability of theemulsion structure, the larger the amount of the electrolyte, thebetter. In this example, the procedure described in Example 1 wasrepeated, except that the coating syrup solution N1-a was prepared afterthe amount of the aqueous KCl electrolyte solution in the precursorsyrup solution N1 of Example 1 was changed, as shown in Table 11-1 toTable 11-3, so as to examine an influence of an increase in amount ofthe aqueous 4% KCl electrolyte solution on the stability of the emulsionstructure. The precursor syrup solution prepared in this example isreferred to as a “syrup number N3” for distinction from that of Example1.

TABLE 11-1 (N3-1) Parts by % by Raw materials weight weight Monomers UVinitiator Irg. 184 0.05 0.044 AA AA 14 12.275 IOA IOA 14 12.275Surfactant SE-10N 16 14.029 Aqueous MPEG550MA 6 5.261 monomerPlasticizer PEG300 2 1.754 Salt solution Humectant Aquadew 22 19.290 4%KCl 4% KCl 39.5 34.634 UV Irg. 2959 0.5 0.438 initiator Total 114.05100.000

TABLE 11-2 (N3-2) Parts by % by Raw materials weight weight Monomers UVIrg. 184 0.05 0.047 initiator AA AA 14 13.139 IOA IOA 14 13.139Surfactant SE-10N 16 15.016 Aqueous MPEG550MA 6 5.631 monomerPlasticizer PEG300 2 1.877 Salt solution Humectant Aquadew 22 20.648 4%KCl 4% KCl 32 30.033 UV Irg. 2959 0.5 0.469 initiator Total 106.55100.000

TABLE 11-3 (N3-3) Parts by % by Raw materials weight weight UV Irg. 1840.05 0.050 initiator AA AA 14 14.134 IOA IOA 14 14.134 MonomersSurfactant SE-10N 16 16.153 Aqueous MPEG550MA 6 6.058 monomerPlasticizer PEG300 2 2.019 Salt solution Humectant Aquadew 22 22.211 4%KCl 4% KCl 24.5 24.735 UV Irg. 2959 0.5 0.505 initiator Total 99.05100.000

In the case of the precursor syrup solution N3-1 described in Table11-1, the average domain diameter measured in the syrup solution beforeadding the water-soluble UV initiator and Irgacure 2959 (supra) wasabout 0.4 μm. The emulsion structure of this syrup solution was stablefor several days.

In the case of the precursor syrup solution N3-2 described in Table11-2, the average domain diameter measured in the syrup solution beforeadding the water-soluble UV initiator was about 0.2 μm. The emulsionstructure of this syrup solution was also stable for several days.

In the case of the precursor syrup solution N3-3 (comparatively smallamount of electrolyte) described in Table 11-3, the average domaindiameter measured in the syrup solution before adding the water-solubleUV initiator was about 15 μm. It was a comparatively unstablemacro-emulsion.

As is apparent from the above results, it has been found that anemulsion syrup, which contains a large amount of an electrolyte aspossible and is stable, is preferred to prepare a conductive adhesivehaving good conductivity.

In the same procedure as in Example 1 except for using each of theprecursor syrup solutions N3-1 to N3-3 in place of the precursor syrupsolution N1, a conductive adhesive sheet of this example was made. Ithas been found that this adhesive sheet has electrical characteristicsand resistance to drying in high level suited for use in the biomedicalelectrode.

Example 4

The same procedure as in Example 1 was repeated. In this examples,however, amino acids are contained as the humectant and an influence ofa change in composition exerted on the characteristics of the resultingconductive adhesive was examined by using a precursor syrup solutioncontaining raw material components that are different from those used inthe aforementioned examples.

Table 12 described below is a table showing the composition of theprecursor syrup solution mixed with the non-reactive surfactant, whichis commercially available from Sigma Co under the product name of Brij97. This precursor syrup solution was a white macro-emulsion.

TABLE 12 Parts by Raw materials weight % by weight Monomer UV Irg. 1840.05 0.039 initiator AA AA 19.6 15.403 IOA IOA 14 11.002 Surfactant Brij97 24 18.861 Aqueous MPEG550MA 11.1 8.723 monomer Plasticizer PEG300 0.00.000 Salt solution Humectant Aquadew 17 13.360 4% KCl 4% KCl 41 32.220UV Irg. 2959 0.5 0.393 initiator Total 127.25 100.000

In the same procedure as in Example 1 except for using the precursorsyrup solution of the above composition in place of the precursor syrupsolution N1, a conductive adhesive sheet of this example was made. Ithas been found that this adhesive sheet has electrical characteristicsand resistance to drying in high level suited for use in the biomedicalelectrode.

Table 13 described below is a table showing the composition of theprecursor syrup solution mixed with aforementioned amino acid PCA-Na(aqueous 50% solution) as the humectant. This precursor syrup solutionwas a white macro-emulsion.

TABLE 13 Parts by Raw materials weight % by weight Monomers UV Irg. 1840.060 0.1 initiator AA AA 11.50 19.0 IOA IOA 16.50 27.3 SurfactantSE-10N 4.00 6.6 Aqueous MPEG550MA 4.00 6.6 monomer Humectant PCA-Na10.20 16.9 Salt solution 4% KCl 4% KCl 13.60 22.5 UV Irg. 2959 0.60 1.0initiator Total 60.46 100.0

In the same procedure as in Example 1 except for using the precursorsyrup solution of the above composition in place of the precursor syrupsolution N1, a conductive adhesive sheet of this example was made. Ithas been found that this adhesive sheet has electrical characteristicsand resistance to drying in high level suited for use in the biomedicalelectrode.

Example 5

In the same procedure as in Example 1 except for using the precursorsyrup solution having the composition described in Table 14 in place ofthe precursor syrup solution N1, a conductive adhesive (adhesive sheet)of this example was made. The precursor syrup solution having thecomposition described below was an opaque emulsion and the adhesive as afinal product obtained after curing was also an opaque conductiveadhesive.

TABLE 14 Parts by Raw materials weight % by weight Monomers UV Irg. 1840.05 0.044 initiator AA AA 14 12.313 IOA IOA 14 12.313 Surfactant SE-10N17.4 15.303 Aqueous MPEG550M 6.4 5.629 monomer A Plasticizer PEG300 2.52.199 Salt solution Humectant Aquadew 22 19.349 4% KCl 4% KCl 37.2532.762 Crosslinking TEGDAM 0.1 0.088 agent Total 113.7 100.000

The adhesive sheet of this example was incorporated into the disposableECG and EKG electrodes for electrocardiography, which were previouslydescribed with reference to FIG. 1 and FIG. 2, to make a biomedicalelectrode. Since a conductor used in the production of this biomedicalelectrode is that disclosed in aforementioned International PublicationNo. WO9741568, please see the description of the related portion of saidpublication with respect to the details.

Subsequently, the aging characteristics after the resulting biomedicalelectrode was put in the sealed pouch and stored at 66° C. for 6 weekswas evaluated in the same procedure as in Example 1. As a result, it hasbeen found that the AAMI characteristics are not deteriorated and thebiomedical electrode has good aging stability. It has also been foundthat the biomedical electrode has good satisfactory AAMI stability evenin case where it was stored under a low-humidity condition of 20° C.-20%RH outside a sealed pouch for 30 days.

Example 6

In this example, a conductive adhesive (adhesive sheet) of this examplewas made in the same procedure as in Example 1 except for using theprecursor syrup solution having the composition described in Table 15below in place of the precursor syrup solution N1.

TABLE 15 Parts by % by Raw materials weight weight Monomers Oil-solubleIrg. 184 0.05 0.036 photo- initiator AA AA 14 10.142 IOA IOA 14 10.142Surfactant E-mal E-70C 9 6.520 Hydrophilic MPEG550MA 8 5.795 monomerPlasticizer PEG300 3 2.173 Cross- Cross- TEGDMA 0.1 0.072 linkinglinking agent agent Salt solution Humectant Aquadew 50 36.221 4% KCl KCl1.39 1.007 Distilled DI water 38 27.528 water Water- Irg. 2959 0.5 0.362soluble photo- initiator Total 138.04 100.000

In this example, to obtain the better resistance to drying, about 36% byweight or more of the amino acid humectant was contained in theprecursor syrup solution and the non-reactive surfactant capable ofexhibiting a higher surface active force than that of the surfactantused in Example 1, aforementioned Enal E-70C™ (sodium polyoxyethylenealkyl(C10-C16) ether sulfate) was used in place of the above surfactantused in Example 1. By using this surfactant, a stable mini-emulsionsyrup could be obtained when the amount of the surfactant is from 5 to8% by weight.

The adhesive sheet of the present invention was incorporated into thestud type electrode described previously with reference to FIG. 3 tomake a biomedical electrode.

In the same procedure as in Example 1, the characteristics of theresulting biomedical electrode was evaluated. As a result, themeasurement results described in Table 16 and Table 17 below wereobtained.

TABLE 16 Aging stability of a stud type biomedical electrode in a sealedpouch at 66° C. AAMI Tests specification Initial 3 weeks 6 weeks DCoffset 100 mV or less −0.1 mV −0.3 mV  0.0 mV potential AC impedance2000 Ω or less 270 Ω 353 Ω 237 Ω Offset potential 100 mV or less 12.3 mV14.6 mV 16.6 mV after 5 seconds have passed since defibrillationRecovery speed 0 to −1.0 mV/s −0.3 mV/s −0.4 mV/s −0.4 mV/s after 5seconds have passed since defibrillation Adhesion 100 to 400 g 150 g 210g 200 g strength of per each per each per each per each stud typeelectrode electrode electrode electrode electrode

TABLE 17 Resistance to drying of a stud type biomedical electrode under20° C.-20% RH outside a sealed pouch AAMI Tests specification Initial 20days 30 days DC offset 100 mV or less −0.1 mV −0.7 mV  2.5 mV potentialAC impedance 2000 Ω or less 270 Ω 442 Ω 675 Ω Offset potential 100 mV orless 12.3 mV 14.8 mV 19.7 mV after 5 seconds have passed sincedefibrillation Recovery speed 0 to −1.0 mV/s −0.3 mV/s −0.4 mV/s −0.4mV/s after 5 seconds have passed since defibrillation Adhesion 100 to400 g 150 g 150 g 120 g strength of stud per each per each per each pereach type electrode electrode electrode electrode electrode

As is apparent from the evaluation results described in Table 16 andTable 17 described above, the biomedical electrode made from theconductive adhesive of this example was hardly dried and the ACimpedance was inhibited to a low value such as 675 Ω even after dryingunder a low-humidity condition of 20° C.-20% RH for 30 days. Withoutusing the sealed pouch with a zip lock fastener, sufficient AAMIcharacteristics and adhesion strength can be maintained outside thepouch. Furthermore, sufficient AAMI characteristics and adhesionstrength can be maintained even after an aging test.

Effect of the Invention

As described above, according to the present invention, there can beprovided a conductive adhesive, which can enhance the moisturizingeffect to effectively prevent drying, because high-performancehumectants such as amino acids can be contained in the conductiveadhesive and, in that case, the structure can be maintained in theeffective state (e.g. state capable of exhibiting sufficient adhesion).According to the present invention, a high-performance biomedicalelectrode can be provided by using such a conductive adhesive.

What is claimed is:
 1. A conductive adhesive comprising: (A) a firstphase comprising at least one hydrophilic polymer, an aqueouselectrolyte solution and at least one amino acid humectant, and (B) asecond phase comprising at least one hydrophobic adhesive polymer,wherein the first phase is a continuous phase and the second phase is adomain phase dispersed in the first phase, and wherein the domain phasehas an average diameter within a range from about 0.02 μm to about 1 mm.2. The conductive adhesive of claim 1 wherein the amino acid humectanthas at least one member selected from the group consisting oftrimethylbetaine, DL-pyrrolidonecarboxylic acid (PCA) and sodiumDL-pyrrolidonecarboxylate.
 3. The conductive adhesive of claim 1 whereinthe content of the humectant is in the range of about 10 to about 40% byweight based on a total amount of the conductive adhesive.
 4. Theconductive adhesive of claim 1 wherein the content of the humectant isin the range of about 15 to about 37% by weight based on a total amountof the conductive adhesive.
 5. A biomedical electrode comprising anadhesive layer containing a conductive adhesive which comprises: (A) afirst phase comprising at least one hydrophilic polymer and at least oneamino acid humectant; and (B) a second phase comprising at least onehydrophobic adhesive polymer; wherein the first phase is a continuousphase and the second phase is a domain phase dispersed in the firstphase, and the domain phase has an average diameter within a range fromabout 0.02 μm to about 1 mm, and an electrode terminal connected withthe adhesive layer.
 6. A method of forming a conductive adhesivecomprising: (1) subjecting a stock solution containing (a) an aqueousmedium containing a first monomer capable of forming a hydrophilicpolymer by polymerization, an aqueous electrolyte solution and an aminoacid humectant and (b) a second monomer capable of forming a hydrophobicadhesive polymer by polymerization, which is contained in the aqueousmedium, to a partial polymerization treatment so as to form a precursorsyrup solution; and (2) further subjecting the precursor syrup solutionto a polymerization treatment.